Growth in vertebrates is governed by the integration of genetic, hormonal, and nutritional components. The most significant endocrine influence in body growth is the complex regulation of the growth hormone (GH)/insulin-like growth factor-I (IGF-I) axis, and this mechanism appears to be highly conserved among vertebrates. Growth hormone is involved in regulating numerous physiological processes besides somatic growth in fish including immune function, lipid and protein metabolism, osmoregulation, and feeding behavior. In teleost fish, secretion of GH from the pituitary is regulated by sex steroids along with several hypothalamic factors, which act in concert under the influence of the physiological and nutritional state of the animal.
Growth hormone acts directly on target tissue by stimulating mitosis, and indirectly by initiating the production and release of IGF-I, a mitogenic factor produced primarily in the liver. The physiological actions of GH are mediated through its binding to the growth hormone receptor (GH-R), located on the surface of cells in target tissue. We have recently identified two GH-R subtypes (GH-R1 and -R2) in the Mozambique tilapia, one of which (GH-R1) we believe to be the putative receptor for somatolactin (SL-R) and the other (GH-R2), the growth hormone-receptor (GH-R). Somatolactin (SL) is a member of the GH/prolactin family of pituitary peptide hormones, which is present in a variety of teleost species as well as the sturgeon and lungfish, but not in tetrapods. The signal transduction by the GH-R leads to the biological actions evoked by GH. Protein restriction during fasting has been shown to reduce circulating IGF-I and liver IGF-I mRNA levels in several teleost species including the tilapia. Alterations in circulating GH and IGF-I due to disruptions in metabolic rhythms in turn alter the number and post-receptor functions of GH-R through changes in the transcription and translation of the GH-R. Insulin-like growth factor-II (IGF-II) shares a high structural homology with IGF-I, and gene ex
Effects of fasting and re-feeding on GH/IGF-I axis in fish
There have been numerous studies of the effects of partial or total nutrient restriction on the GH/IGF-I axis in teleost fish. One of the first comprehensive studies on the subject was designed to address the mechanisms that underlay growth hormone resistance. This laid the groundwork for future experiments that sought to understand the seemingly contradictory findings that elevated plasma GH in fish is not necessarily correlated with increased growth rate. Instead, increased binding of GH to its hepatic receptor is strongly correlated with seasonal increases ingrowth, regardless of changes in circulating GH. Furthermore, a GH-induced catabolic state during fasting is associated with many factors including lowered circulating and hepatic ex
Interestingly, Menton et al. (2000) observed a decline in IGF-I mRNA levels following a period of fasting in the sea bream that recovered upon re-feeding. On the other hand, they saw no effect of dietary protein levels on IGF-I mRNA transcript levels. The authors also observed no correlation between body growth and hepatic IGF-I mRNA levels. Similarly, food deprivation was shown to increase plasma GH in the goldfish, while subsequent re-feeding reversed the effects. Replacement of fishmeal with plant-derived protein sources in feed produced a significant increase in plasma GH and a decrease in plasma IGF-I following an overnight fast in sea bream. Likewise, fasting resulted in an elevation in plasma GH and a decrease in plasma and hepatic ex
These results indicate that the up-regulation of plasma GH in parallel with decreased plasma IGF-I and liver ex
Mozambique tilapia is known to grow faster in seawater than in fresh water. Nevertheless, previous studies of the effects of fasting have been restricted to the study of fish acclimated to fresh water. In view of the fact that GH plays an important role in seawater acclimation in several euryhaline fishes including the tilapia, fasting in seawater may affect the GH/IGF-I axis of the tilapia more profoundly than in the fish in fresh water.